Electrically powered vehicles are commonly used in urban transportation systems in which the vehicles travel along rails and obtain power either directly from the rails or from the rails in combination with an overhead power line. Such vehicles are commonly provided with two types of braking systems: a "regenerative braking system" in which the electric motor normally driving the vehicle is conditioned during braking to operate as a generator thereby permitting recapture of the vehicle's kinetic energy during braking, and a "friction braking system" which normally involves frictional engagement of a braking member between stationary and rotating members of the vehicle's wheels. Friction braking merely dissipates the vehicle's kinetic energy as heat, and therefore use of friction braking is preferably minimized. Another consideration is to ensure proper braking under poor rail conditions, particularly the low adhesion characteristic of rain-dampened rails.
According to a known system, when an operator aboard an electrically powered urban transit vehicle depresses his brake pedal, he provides a brake demand signals which is initially responded to by the regenerative braking system. The regenerative braking system provides a braking effect in general proportion to the magnitude of the brake demand signal until that signal exceeds a predetermined level which normally corresponds in substance to the limit of the regenerative braking available. At that point, the friction braking system is actuated to provide any additional demand required. In typical applications, regenerative braking associated with a particular powered truck supporting the vehicle may be used to provide braking up to for example, 3.5 miles per hour per second (mph/sec) with no braking from friction sources being used. This will effectively call for adhesion of the powered trucks of around 4.8 mphs, for a typical vehicle. These figures will depend of course in large measure on the adhesion available between the vehicle wheels and the rails and also the distribution of weight throughout the vehicle. On a wet track, regenerative braking might typically be limited to 2.0 mph/sec, the wheels then tending to spin or slide relative to the track with further regenerative braking. To accommodate the sliding associated with poor rail conditions, such a braking system has been equipped with a slide detector which together with associated control circuitry applies essentially anoscillatory brake demand signal to the braking system, causing the regenerative braking system to repeatedly brake until sliding is detected and then reduce braking to a level below the threshold at which sliding occurs. Because the brake demand signal has been reduced below the level required for actuation of the friction braking system, only regenerative braking is available, and the braking capability of the vehicle is very significantly impaired. If an emergency situation arises in which the operator requires additional braking, such as a vehicle stopped on the rails, the operator will normally have available to him a track brake which physically engages the rails of the track to provide emergency braking. Use of such a track brake is very undesirable because of attendant damage to the track.
As an object of the invention in the context of a transportation system of the type described is to provide a vehicle with an improved braking system which can be adapted if desired to maximize regeneration under good rail conditions and to provide more effective, balanced braking under poor, slippery rail conditions.